To meet the recent demand for higher integration in integrated circuits, pattern formation to a finer feature size is required. Acid-catalyzed chemically amplified resist compositions are most often used in forming resist patterns with a feature size of 0.2 μm or less. High-energy radiation such as UV, deep-UV or electron beam (EB) is used as the light source for exposure of these resist compositions. In particular, while EB lithography is utilized as the ultra-fine microfabrication technique, it is also indispensable in processing a photomask blank to form a photomask for use in semiconductor device fabrication.
The resist compositions for use in photolithography include positive tone compositions wherein a pattern is formed after the exposed region is dissolved and negative tone compositions wherein the exposed region is left to form a pattern. A choice may be made depending on the desired resist pattern structure.
In general, the EB lithography is by writing an image with EB, without using a mask. In the case of positive resist, those regions of a resist film other than the regions to be retained are successively irradiated with EB having a minute area. In the case of negative resist, those regions of a resist film to be retained are successively irradiated with EB. The operation of successively scanning all finely divided regions on the work surface takes a long time as compared with full wafer exposure through a photomask. In order to avoid any decline of throughput, the resist film must be highly sensitive. Because of the long image-writing time, there is a likelihood of a difference arising between the initially written portion and the later written portion. Thus the stability with time of exposed regions in vacuum is one of important performance requirements. One of the important applications of chemically amplified resist material resides in processing of photomask blanks. Some photomask blanks have a surface material that can have an impact on the pattern profile of the overlying chemically amplified resist film, such as a layer of a chromium compound, typically chromium oxide deposited on a photomask substrate. For high resolution and profile retention after etching, it is one important performance factor to maintain the pattern profile of resist film rectangular independent of the type of substrate.
The control of resist sensitivity and pattern profile as mentioned above has been improved by a proper selection and combination of resist material-constituting components and processing conditions. One outstanding improvement is directed to the diffusion of acid that largely affects the resolution of a chemically amplified resist film. In processing of photomasks, it is required that the profile of a resist pattern formed as above do not change with a lapse of time from the end of exposure to PEB. The major cause of such a change with time is diffusion of an acid generated upon exposure. The problem of acid diffusion has been widely studied not only in the field of photomask processing, but also in the field of general resist films because it has a significant impact on sensitivity and resolution.
Patent Documents 1 and 2 describe acid generators capable of generating bulky acids for controlling acid diffusion and reducing roughness. Since these acid generators are still insufficient in control of acid diffusion, it is desired to have an acid generator with more controlled diffusion.
Patent Document 3 discloses a resist composition comprising a base resin to which a sulfonic acid generated upon light exposure is bound so that the acid diffusion is controlled. This approach of controlling acid diffusion by binding recurring units capable of generating acid upon exposure to a base polymer is effective in forming a pattern with minimal LER. However, a problem arises with respect to the solubility in organic solvent of the base polymer having bound therein recurring units capable of generating acid upon exposure, depending on the structure and proportion of such recurring units.
Polymers comprising a major proportion of aromatic structure having an acidic side chain, for example, polyhydroxystyrene have been widely used in resist materials for the KrF excimer laser lithography. These polymers are not used in resist materials for the ArF excimer laser lithography since they exhibit strong absorption at a wavelength around 200 nm. These polymers, however, are expected to form useful resist materials for the EB and EUV lithography for forming patterns of finer size than the processing limit of ArF excimer laser because they offer high etching resistance.
Often used as the base polymer in positive resist compositions for EB and EUV lithography is a polymer having an acidic functional group on phenol side chain masked with an acid labile protective group wherein the acid labile protective group is deprotected by the catalysis of an acid generated from a photoacid generator upon exposure to high-energy radiation so that the polymer may become soluble in alkaline developer. Typical of the acid labile protective group are tertiary alkyl, tert-butoxycarbonyl, and acetal groups. On use of protective groups requiring a relatively low level of activation energy for deprotection such as acetal groups, a resist film having a high sensitivity is advantageously obtainable. However, if the diffusion of generated acid is not fully controlled, deprotection reaction can occur even in the unexposed regions of the resist film, giving rise to problems like degradation of line edge roughness (LER) and a lowering of in-plane uniformity of pattern line width (CDU).
Patent Document 4 describes a resist composition comprising a resin comprising recurring units having an acetal group and a sulfonium salt capable of generating an acid having a high pKa such as fluoroalkanesulfonic acid. Regrettably, the pattern obtained therefrom has substantial LER. This is because the acid strength of fluoroalkanesulfonic acid is too high for the deprotection of an acetal group requiring a relatively low level of activation energy for deprotection. So, even if acid diffusion is controlled, deprotection reaction can occur in the unexposed region with a minor amount of acid diffused thereto.
The problem that due to acid diffusion, undesired reaction occurs in the unexposed region to incur roughness degradation is common to negative resist compositions for EB lithography and resist compositions for EUV lithography. In the case of negative resist compositions, undesirable crosslinking reaction takes place in the unexposed region due to the acid diffused thereto, and as a result, patterns with noticeable LER are formed.